The demand for higher capacity in wireless communication systems has persisted over the years. As a consequence the various vendors and other actors have produced wireless communication systems with continuously increasing capacity. One example is the wireless communication systems based on the standards produced by the 3rd Generation Partnership Project (3GPP, see e.g. www.3gpp.org). Here, the early GSM-systems (Global System for Mobile Communication) have a considerably lower capacity than the more resent Long Term Evolution (LTE) system and LTE-Advanced systems.
LTE-Advanced maintains the basic LTE approach to a large extent. Enhancements include carrier aggregation, higher order MIMO schemes in Downlink and Uplink, enhanced Uplink transmission, Coordinated MultiPoint (CoMP) transmission/reception and the support of relays.
A relay (i.e. a relay node) may be seen as an intelligent repeater. Typically, information is forwarded (possibly in an altered form) by the relay node from a donor node to a user equipment served by the relay node. Conversely, information is typically forwarded (possibly in an altered form) by the relay node from the user equipment to the donor node. Generally, a relay node is typically used to enhance coverage and capacity in a particular area. The particular area may e.g. be a poorly covered area in a cell served by an access node acting as a donor node for the relay node, e.g. an area in radio shadow or an area at the edge of the cell.
LTE-Advance defines at least two different types of relays, Type 1 relay nodes and Type 2 relay nodes. The background discussion in this section may relate to Type 1 relay nodes. However, embodiments of the present solution are equally applicable to other relay nodes, e.g. to Type 2 relay nodes or similar.
Type 1 relay nodes control their cells with their own cell identity, including transmission of synchronization channels and reference symbols. Type 1 relay nodes appear as a Release 8 eNB to Release 8 UEs, which ensures backward-compatible operation. Type 1 relay nodes are e.g. defined in Release 10 of the 3 GPP standards.
FIG. 1 is a schematic illustration of a LTE system comprising a Type 1 relay node (RN) 16 and an eNodeB (eNB) 12 acting as a donor node for the relay node 16. The eNB 12 communicates with a User Equipment (UE) 11 via a first access interface Uu, and the relay node 16 communicates with UE 15 via a second access interface Uu. The relay node 16 and the eNB 12 communicate via a backhaul interface Un. The donor eNB 12 may send downlink user information to and receive uplink user information from the relay node 16 via the backhaul interface Un. The user information is typically forwarded (possibly in an altered form) by the relay node 16 from the eNB 12 to the UE 11, and from the UE 11 to the eNB 12. The eNB 12 may also send control information or similar to the relay node 16 via the backhaul interface Un. The control information may e.g. define the properties of the communication occurring between the relay node 16 and the UE 15.
A relay node is commonly referred to as an inband relay when the communication on the backhaul interface (e.g. Un) on one hand and the access interfaces (e.g. Uu) on the other hand are performed within the same frequency band. Conversely, a relay node is commonly referred to as an outband relay when the communication on the backhaul interface on one hand and the access interfaces on the other hand are performed on separated frequency bands. The discussion in this background section is mainly related to inband relay nodes. However, embodiments of the present solution are equally applicable to outband relay nodes and other relays.
The attention is now turned to FIG. 2. Here, it is assumed that the inband relay node 16 transmits downlink information to the UE 15 via the access interface Uu. It is also assumed that the UE 11 receives downlink information from the eNB 12. It is also assumed that the UE 11 is close to the relay node 16 such that the UE 11 experiences much more interference from the relay node 16 than other UEs being further away from the relay node 16. The interferences have been illustrated by two dashed lines in FIG. 2 extending from the relay node 16 to the UE 11.
In FIG. 3 it is assumed that the UE 11 transmits uplink information to the eNB 12. It is also assumed that the UE 11 is close to the inband relay node 16. The transmitting UE 11 will then create much more interference in the relay node 16 than other UEs further away from the relay node 16. The interferences have been illustrated by two dashed lines in FIG. 3 extending from the UE 11 to the relay node 16.
The expression “close” used in this text should be interpreted such that a UE is receiving a signal from a relay node that is stronger than a predetermined threshold. The effect, when a UE is receiving a signal from a relay node that is stronger than a predetermined threshold, is that a radio signal transmitted from the relay node may cause high interference in the UE in question, or conversely that a radio signal transmitted from the UE may cause high interference in the relay node in question. For example the predetermined threshold may be indicative of the difference (e.g. measured by the UE) between the signal strength of the relay node and the signal strength of the access node serving the UE. The predetermined threshold may e.g. be a difference of less than 3 dB, or less than 6 dB or less than 9 dB. A high interference may e.g. occur if the UE is located within one or a few hundred meters from the relay node, particularly if the UE is in line of sight of the relay node. However, this may not be the case if the UE is located in a “radio shadow” even if the UE is less than a hundred meters from the relay node. Radio shadow may e.g. occur inside an elevator cage made of metal or behind a thick wall made of solid stone or heavily reinforced concrete or some other material that attenuates or stops propagation of radio waves.